The most common method is to apply them a little heavier then normal, then spray, then apply again if needed.
This saves over-applying, then sealing the model to find out you over applied and worrying about fingerprints and such if someone touches your model covered in pastel chalk.
And I'm sorry to say that the fixative was not required. It's designed specifically for artists, and for modelers a clear coat will work the same. I mean, an artist could spray a coat of Polly S Flat Clear on their picture and it would work fine.